1. Field of the Invention
The present invention relates to a battery pack to be used as a power supply for, for example, digital still cameras, personal computers, video cameras, cellular phones, etc., and more specifically to a battery pack provided with a protection circuit which shuts off discharge in case when external plus and minus terminals of such battery pack are short-circuited.
2. Description of Related Art
In a conventional battery pack including, for example, secondary batteries, if a large discharge current in excess of the rated discharge current is forced to flow, it is possible that performance of the secondary batteries degrade and decrease their discharge capacity or damage the secondary battery themselves.
In order to solve such problems, it is typical to provide a protection circuit inside the battery pack for protecting a primary or secondary battery from such overcurrent discharge by turning OFF (or opening), for example, a discharging control switch in order to shut off the discharge current whenever a current in excess of a predetermined current value flows from the battery pack over a predetermined time, thereby protecting the primary or secondary battery from overcurrent.
In addition to the above protection circuit, it is general to improve safety by providing a recessed portion on the outer surface of the pack and disposing external terminals of the battery pack in the recessed portion so that the external terminals do not easily make electrical contact with any external metal. However, if it is constructed to dispose the external terminals in the recessed portion, assembling processes are increased and adversely affecting the working efficiency and in turn increasing the production cost as compared to the case of disposing the external terminals on the surface of the battery pack. As a matter of fact, such conventional approach does not basically provide protection of the primary or secondary battery.
FIG. 29 shows one example of such conventional protection circuit. In FIG. 29, an internal battery as accommodated inside the battery pack (referred to as a battery cell 1 hereinafter) is connected to a battery cell positive terminal 3 of a protection circuit 2 at the positive side of the battery cell 1 while connecting to a battery cell negative terminal 4 at the negative side of the battery cell 1.
The battery cell positive terminal 3 is connected to an external plus terminal 5 and also connected to a positive side power supply terminal 8 of a control IC 7 by way of a junction 6.
On the other hand, the battery cell negative terminal 4 is connected to a negative side power supply terminal 10 of the control IC 7 and a resistor 11 by way of a junction 9.
The resistor 11 is connected to an anode side of a diode 12 and also a discharging control switch 13. The diode 12 and the discharging control switch 13 are connected in parallel with each other and the cathode side of the diode 12 and the other terminal of the discharging control switch 13 are connected to a cathode side of a diode 14 and a charging control switch 15.
The diode 14 and the charging control switch 15 are connected in parallel. The other end of the charging control switch 15 and the anode side of the diode 14 are connected to an external minus terminal 17 of the protection circuit 2 by way of a junction 16.
For example, voltage detectors 18, 19, an operational amplifier 20, a resistor 21, a switch 22, etc. are disposed inside the control IC 7. The positive side power supply terminal 8 is connected to the negative side power supply terminal 10 by way of the voltage detector 18.
The voltage detector 18 is also connected to the voltage detector 19 and the resistor 21. The resistor 21 is connected to the switch 22 which is connected to the voltage detector 19 and an overcurrent voltage detection terminal 23.
The overcurrent voltage detection terminal 23 is connected to the external minus terminal 17 of the protection circuit 2 by way of the junction 16.
The voltage detector 18 detects the voltage between the battery cell positive terminal 3 and the battery cell negative terminal 4, i. e., between the positive side and the negative side of the battery cell 1. On the other hand, the voltage detector 19 detects the entire voltage across the resistor 11, the diodes 12, 14, the discharging control switch 13 and the charging control switch 15 which are connected between the battery cell negative terminal 4 and the external minus terminal 17.
Voltage detection results detected by these voltage detectors 18, 19 are supplied to the operational amplifier 20 which controls the switch 22 in response to the abovementioned voltage detection results.
In case of charging and discharging the battery pack, the discharging control switch 13 and the charging control switch 15 are designed to be controlled in response to control signals from the control IC 7.
It is to be noted here that both discharging control switch 13 and the charging control switch 15 are in the ON (or closed) condition in case when the battery pack is in the normal condition, i.e., when the battery cell 1 is discharging its current into a load (not shown) connected between the external plus terminal 5 and the external minus terminal 17 and in case of charging the battery cell 1 by a charger (not shown) connected between the external plus terminal 5 and the external minus terminal 17.
In other words, under the normal condition when both of the discharging control switch 13 and the charging control switch 15 are ON (closed), discharge and charge operations can be performed freely.
When the voltage of the battery cell 1 is equal to or higher than the predetermined voltage, i.e., in the fully charged condition, the discharging control switch 13 remains in the ON (closed) condition but the charging control switch 15 becomes OFF (open) condition in response to a charging control signal 24 from the control IC 7.
When the charging control switch 15 is in the OFF (open) condition as mentioned above, the diode 14 allows discharging into the load while disabling to charge the battery cell 1, thereby protecting the battery cell 1 from over-charging.
In case when the voltage of the battery cell 1 decreases below the predetermined voltage, i.e., in the over-discharging condition, the charging control switch 15 is in the ON (closed) condition while the discharging control switch 13 is turned OFF (open) by a discharging control signal 25 from the control IC 7.
In the OFF (open) condition of the discharging control switch 13 as mentioned above, the function of the diode 12 allows charging of the battery cell 1 but disabling to discharge into the load, thereby protecting the battery cell 1 from over-discharge.
Moreover, in case when a low resistor or a conductor such as, for example, an electrical wire is connected to short-circuit between the external plus terminal 5 and the external minus terminal 17 from outside of the battery pack, the charging control switch 15 is in the ON (closed) condition while the discharging control switch 13 is in the OFF (open) condition, thereby not discharging into the load.
As described above, in case when the external plus terminal 5 and the external minus terminal 17 are short-circuited in the conventional protection circuit, it is determined to be overcurrent if discharge current in excess of, for example, approximately 4 A flows over about 0.01 second, thereby shutting off the discharge current by turning OFF (open) the discharging control switch 13.
A condition to recover the condition protected from the overcurrent, i.e., the ON (closed) condition of the discharging control switch 13 from the OFF (open) condition is that the resistance externally connected to external terminals of the battery pack increases to, for example, approximately 100 kΩ to 200 MΩ or larger.
Accordingly, in case when a trouble occurs in the internal circuit of an electronic apparatus or the like to which the battery pack is connected and the resistance of the electronic apparatus becomes, for example, 0.8 Ω or lower, the discharging control switch 13 is turned OFF (open) and such condition is maintained.
As for a circuit for protecting the primary or secondary battery from overcurrent, proposed is an overcurrent protection circuit for a battery in which switch means is turned off, for example, upon detection by current detection means of a current larger than a predetermined value flowing out of the battery and automatically returning the switch means after lapse of a predetermined time which is automatically adjusted in substantially proportion to the current value detected by the current detection means (see Patent Document 1).
According to the prior art as disclosed in the Patent Document 1, in case when a current larger than a predetermined value flows out of the battery for a period longer than a predetermined time, switch means is turned off and allows the discharge current to flow.
Also disclosed is a provision of a movable shield plate corresponding to the charging terminal and a movable shield plate corresponding to the power supply terminal, thereby preventing any serious trouble such as overheating, catching fire, etc. (see Patent Document 2).                Patent Document 1: Japanese patent No.3272104        Patent Document 2: Japanese non-examined patent publication No. H9-320554        
However, in case of repetitively connecting and disconnecting a load between the external terminals of the battery pack in the conventional overcurrent protection circuits, for example, in case of connecting a metal chain of a necklace chain or the like between the external terminals of the battery pack (referred to as chain-short below), the discharging control switch 13 repeats ON and OFF conditions, thereby repeating a large current discharge and decreasing the discharge capacity of the battery pack, causing a trouble in the primary or secondary battery, smoking from the battery pack, making the plastic case of the battery pack non-usable because of partial melting or distortion by the heated metal chain or the like. Moreover, the user may be burned by the heat. In order to avoid such trouble, it is normal in some battery packs to recommend in their instruction manuals or the like that the user mounts a plastic terminal cover for protecting the external terminals of the battery pack or warns the user not to connect a metal chain of a necklace or the like.
In case of connecting a metal chain of a necklace or the like between the external terminals of the battery pack, there causes repetitive connection and disconnection of the load because, although the metal chain appears to be mechanically always connected between the external terminals, a large current develops oxidation or the like on the contacting surfaces between adjacent rings of the chain, thereby electrically repeating connection and disconnection, i.e., substantially 0 Ω and substantially ∞Ω.
Accordingly, in case when the battery pack and a metal chain of a necklace or the like are put together in a bag or the like, the metal chain makes contact between the external terminals of the battery pack, thereby causing a trouble in the battery pack depending on situations.
As a concrete example of connecting such metal chain or the like between the external terminals of the battery pack, shown in FIG. 30 is the relationship between the amplitude of the discharge current, the surface temperature of the external plus terminal 5 (positive terminal temperature), the surface temperature of the external minus terminal 17 (negative terminal temperature) and the surface temperature of the battery pack (cell surface temperature) when a Kihei-type iron chain is connected between the external terminals of the battery pack.
As apparent from FIG. 30, in case of connecting a metal chain or the like between the external terminals of the battery pack, it is understood that overcurrent discharge is repeated, thereby particularly increasing the surface temperature of the external plus terminal (positive terminal temperature).
FIG. 31 shows a graph of discharging characteristics for measuring a discharge capacity of a battery pack before and after the repetitive overcurrent discharge.
As apparent from FIG. 31, in case of connecting a metal chain or the like between the external terminals of a battery pack, it is understood that the discharge capacity decreases after the chain-short test as compared to before the chain-short test.
In the prior art as disclosed in the above Patent Document 1, in case when, for example, a load substantially equal to the resistance of the battery is repetitively connected, overcurrent flows repetitively out of the battery, thereby possibly causing a problem to damage the primary or secondary battery in the battery back.
On the other hand, the prior art as disclosed in the Patent Document 2 is complex in mechanical construction, difficult to manufacture and high in production cost.
Accordingly, the conventional battery packs have problems to be solved so that, even in case when a load is repetitively connected and disconnected between the external terminals of the battery pack, the primary or secondary battery in the battery pack is protected not to cause any trouble and yet the mechanical construction is simple.